16.1. select — Waiting for I/O completion

This module provides access to the select() and poll() functions available in most operating systems, epoll() available on Linux 2.5+ and kqueue() available on most BSD. Note that on Windows, it only works for sockets; on other operating systems, it also works for other file types (in particular, on Unix, it works on pipes). It cannot be used on regular files to determine whether a file has grown since it was last read.

The module defines the following:

exception select.error

The exception raised when an error occurs. The accompanying value is a pair containing the numeric error code from errno and the corresponding string, as would be printed by the C function perror().

select.epoll([sizehint=-1])

(Only supported on Linux 2.5.44 and newer.) Returns an edge polling object, which can be used as Edge or Level Triggered interface for I/O events; see section Edge and Level Trigger Polling (epoll) Objects below for the methods supported by epolling objects.

버전 2.6에 추가.

select.poll()

(Not supported by all operating systems.) Returns a polling object, which supports registering and unregistering file descriptors, and then polling them for I/O events; see section Polling Objects below for the methods supported by polling objects.

select.kqueue()

(Only supported on BSD.) Returns a kernel queue object; see section Kqueue Objects below for the methods supported by kqueue objects.

버전 2.6에 추가.

select.kevent(ident, filter=KQ_FILTER_READ, flags=KQ_EV_ADD, fflags=0, data=0, udata=0)

(Only supported on BSD.) Returns a kernel event object; see section Kevent Objects below for the methods supported by kevent objects.

버전 2.6에 추가.

select.select(rlist, wlist, xlist[, timeout])

This is a straightforward interface to the Unix select() system call. The first three arguments are sequences of 〈waitable objects〉: either integers representing file descriptors or objects with a parameterless method named fileno() returning such an integer:

  • rlist: wait until ready for reading

  • wlist: wait until ready for writing

  • xlist: wait for an 《exceptional condition》 (see the manual page for what your system considers such a condition)

Empty sequences are allowed, but acceptance of three empty sequences is platform-dependent. (It is known to work on Unix but not on Windows.) The optional timeout argument specifies a time-out as a floating point number in seconds. When the timeout argument is omitted the function blocks until at least one file descriptor is ready. A time-out value of zero specifies a poll and never blocks.

The return value is a triple of lists of objects that are ready: subsets of the first three arguments. When the time-out is reached without a file descriptor becoming ready, three empty lists are returned.

Among the acceptable object types in the sequences are Python file objects (e.g. sys.stdin, or objects returned by open() or os.popen()), socket objects returned by socket.socket(). You may also define a wrapper class yourself, as long as it has an appropriate fileno() method (that really returns a file descriptor, not just a random integer).

참고

File objects on Windows are not acceptable, but sockets are. On Windows, the underlying select() function is provided by the WinSock library, and does not handle file descriptors that don’t originate from WinSock.

select.PIPE_BUF

Files reported as ready for writing by select(), poll() or similar interfaces in this module are guaranteed to not block on a write of up to PIPE_BUF bytes. This value is guaranteed by POSIX to be at least 512. Availability: Unix.

버전 2.7에 추가.

16.1.1. Edge and Level Trigger Polling (epoll) Objects

http://linux.die.net/man/4/epoll

eventmask

Constant

Meaning

EPOLLIN

Available for read

EPOLLOUT

Available for write

EPOLLPRI

Urgent data for read

EPOLLERR

Error condition happened on the assoc. fd

EPOLLHUP

Hang up happened on the assoc. fd

EPOLLET

Set Edge Trigger behavior, the default is Level Trigger behavior

EPOLLONESHOT

Set one-shot behavior. After one event is pulled out, the fd is internally disabled

EPOLLRDNORM

Equivalent to EPOLLIN

EPOLLRDBAND

Priority data band can be read.

EPOLLWRNORM

Equivalent to EPOLLOUT

EPOLLWRBAND

Priority data may be written.

EPOLLMSG

Ignored.

epoll.close()

Close the control file descriptor of the epoll object.

epoll.fileno()

Return the file descriptor number of the control fd.

epoll.fromfd(fd)

Create an epoll object from a given file descriptor.

epoll.register(fd[, eventmask])

Register a fd descriptor with the epoll object.

참고

Registering a file descriptor that’s already registered raises an IOError – contrary to Polling Objects’s register.

epoll.modify(fd, eventmask)

Modify a register file descriptor.

epoll.unregister(fd)

Remove a registered file descriptor from the epoll object.

epoll.poll([timeout=-1[, maxevents=-1]])

Wait for events. timeout in seconds (float)

16.1.2. Polling Objects

The poll() system call, supported on most Unix systems, provides better scalability for network servers that service many, many clients at the same time. poll() scales better because the system call only requires listing the file descriptors of interest, while select() builds a bitmap, turns on bits for the fds of interest, and then afterward the whole bitmap has to be linearly scanned again. select() is O(highest file descriptor), while poll() is O(number of file descriptors).

poll.register(fd[, eventmask])

Register a file descriptor with the polling object. Future calls to the poll() method will then check whether the file descriptor has any pending I/O events. fd can be either an integer, or an object with a fileno() method that returns an integer. File objects implement fileno(), so they can also be used as the argument.

eventmask is an optional bitmask describing the type of events you want to check for, and can be a combination of the constants POLLIN, POLLPRI, and POLLOUT, described in the table below. If not specified, the default value used will check for all 3 types of events.

Constant

Meaning

POLLIN

There is data to read

POLLPRI

There is urgent data to read

POLLOUT

Ready for output: writing will not block

POLLERR

Error condition of some sort

POLLHUP

Hung up

POLLNVAL

Invalid request: descriptor not open

Registering a file descriptor that’s already registered is not an error, and has the same effect as registering the descriptor exactly once.

poll.modify(fd, eventmask)

Modifies an already registered fd. This has the same effect as register(fd, eventmask). Attempting to modify a file descriptor that was never registered causes an IOError exception with errno ENOENT to be raised.

버전 2.6에 추가.

poll.unregister(fd)

Remove a file descriptor being tracked by a polling object. Just like the register() method, fd can be an integer or an object with a fileno() method that returns an integer.

Attempting to remove a file descriptor that was never registered causes a KeyError exception to be raised.

poll.poll([timeout])

Polls the set of registered file descriptors, and returns a possibly-empty list containing (fd, event) 2-tuples for the descriptors that have events or errors to report. fd is the file descriptor, and event is a bitmask with bits set for the reported events for that descriptor — POLLIN for waiting input, POLLOUT to indicate that the descriptor can be written to, and so forth. An empty list indicates that the call timed out and no file descriptors had any events to report. If timeout is given, it specifies the length of time in milliseconds which the system will wait for events before returning. If timeout is omitted, negative, or None, the call will block until there is an event for this poll object.

16.1.3. Kqueue Objects

kqueue.close()

Close the control file descriptor of the kqueue object.

kqueue.fileno()

Return the file descriptor number of the control fd.

kqueue.fromfd(fd)

Create a kqueue object from a given file descriptor.

kqueue.control(changelist, max_events[, timeout]) → eventlist

Low level interface to kevent

  • changelist must be an iterable of kevent objects or None

  • max_events must be 0 or a positive integer

  • timeout in seconds (floats possible); the default is None, to wait forever

16.1.4. Kevent Objects

https://www.freebsd.org/cgi/man.cgi?query=kqueue&sektion=2

kevent.ident

Value used to identify the event. The interpretation depends on the filter but it’s usually the file descriptor. In the constructor ident can either be an int or an object with a fileno() function. kevent stores the integer internally.

kevent.filter

Name of the kernel filter.

Constant

Meaning

KQ_FILTER_READ

Takes a descriptor and returns whenever there is data available to read

KQ_FILTER_WRITE

Takes a descriptor and returns whenever there is data available to write

KQ_FILTER_AIO

AIO requests

KQ_FILTER_VNODE

Returns when one or more of the requested events watched in fflag occurs

KQ_FILTER_PROC

Watch for events on a process id

KQ_FILTER_NETDEV

Watch for events on a network device [not available on Mac OS X]

KQ_FILTER_SIGNAL

Returns whenever the watched signal is delivered to the process

KQ_FILTER_TIMER

Establishes an arbitrary timer

kevent.flags

Filter action.

Constant

Meaning

KQ_EV_ADD

Adds or modifies an event

KQ_EV_DELETE

Removes an event from the queue

KQ_EV_ENABLE

Permitscontrol() to returns the event

KQ_EV_DISABLE

Disablesevent

KQ_EV_ONESHOT

Removes event after first occurrence

KQ_EV_CLEAR

Reset the state after an event is retrieved

KQ_EV_SYSFLAGS

internal event

KQ_EV_FLAG1

internal event

KQ_EV_EOF

Filter specific EOF condition

KQ_EV_ERROR

See return values

kevent.fflags

Filter specific flags.

KQ_FILTER_READ and KQ_FILTER_WRITE filter flags:

Constant

Meaning

KQ_NOTE_LOWAT

low water mark of a socket buffer

KQ_FILTER_VNODE filter flags:

Constant

Meaning

KQ_NOTE_DELETE

unlink() was called

KQ_NOTE_WRITE

a write occurred

KQ_NOTE_EXTEND

the file was extended

KQ_NOTE_ATTRIB

an attribute was changed

KQ_NOTE_LINK

the link count has changed

KQ_NOTE_RENAME

the file was renamed

KQ_NOTE_REVOKE

access to the file was revoked

KQ_FILTER_PROC filter flags:

Constant

Meaning

KQ_NOTE_EXIT

the process has exited

KQ_NOTE_FORK

the process has called fork()

KQ_NOTE_EXEC

the process has executed a new process

KQ_NOTE_PCTRLMASK

internal filter flag

KQ_NOTE_PDATAMASK

internal filter flag

KQ_NOTE_TRACK

follow a process across fork()

KQ_NOTE_CHILD

returned on the child process for NOTE_TRACK

KQ_NOTE_TRACKERR

unable to attach to a child

KQ_FILTER_NETDEV filter flags (not available on Mac OS X):

Constant

Meaning

KQ_NOTE_LINKUP

link is up

KQ_NOTE_LINKDOWN

link is down

KQ_NOTE_LINKINV

link state is invalid

kevent.data

Filter specific data.

kevent.udata

User defined value.